AD8614/AD8644
Rev. B | Page 9 of 16
THEORY OF OPERATION
The AD8614/AD8644 are processed using Analog Devices high
voltage, extra fast complementary bipolar (HV XFCB) process.
This process includes trench-isolated transistors that lower
parasitic capacitance.
Figure 26 shows a simplified schematic of the AD8614/AD8644.
The input stage is rail-to-rail, consisting of two complementary
differential pairs, one NPN pair and one PNP pair. The input
stage is protected against avalanche breakdown by two back-to-
back diodes. Each input has a 1.5 kΩ resistor that limits input
current during overvoltage events and furnishes phase reversal
protection if the inputs are exceeded. The two differential pairs
are connected to a double-folded cascode. This is the stage in
the amplifier with the most gain. The double-folded cascode
differentially feeds the output stage circuitry. Two complemen-
tary common emitter transistors are used as the output stage.
This allows the output to swing to within 125 mV from each rail
with a 10 mA load. The gain of the output stage, and thus the
open-loop gain of the op amp, depends on the load resistance.
The AD8614/AD8644 have no built-in short-circuit protection.
The short-circuit limit is a function of high current roll-off of
the output stage transistors and the voltage drop over the
resistor shown on the schematic at the output stage. The voltage
over this resistor is clamped to one diode during short-circuit
voltage events.
OUTPUT SHORT-CIRCUIT PROTECTION
To achieve a wide bandwidth and high slew rate, the output of
the AD8614/AD8644 is not short-circuit protected. Shorting
the output directly to ground or to a supply rail can destroy the
device. The typical maximum safe output current is 70 mA.
In applications where some output current protection is needed,
but not at the expense of reduced output voltage headroom, a
low value resistor in series with the output can be used. This is
shown in
Figure 25. The resistor is connected within the
feedback loop of the amplifier so that if V
OUT
is shorted to
ground and V
IN
swings up to 18 V, the output current does not
exceed 70 mA.
For 18 V single-supply applications, resistors less than 261 Ω are
not recommended.
AD86x4
V
IN
261Ω
V
OUT
18
06485-026
Figure 25. Output Short-Circuit Protection
CC
– +
1.5kΩ
V
EE
V
OUT
V
CC
V
CC
1.5kΩ
06485-025
Figure 26. Simplified Schematic
